Field
Various compressor systems may benefit from a lightweight compressor piston with a circumferential opening. For example, a piston type, positive displacement, reciprocating gas compressor may benefit from a compressor piston that reduces reciprocating weight.
Description of the Related Art
Manufacturers of piston type reciprocating gas compressors may produce pistons which are very heavy and their weight may prohibit them from achieving all of their goals for the devices. These pistons may be restrictive due to being heavy.
A double-acting compressor piston may be defined as a positive displacement piston that compresses gas on both the crank end (CE) and head end (HE) of the piston inside of a cylinder with a single main bore. A CE compression chamber is located closest to the crankshaft and a HE compression chamber is located closest to the head-end (farthest from the crankshaft). Both CE and HE compression chambers reside in the same cylinder. The cylinder has one main bore. When the piston moves towards the CE it compresses gas on the CE while creating suction on the HE. When the piston moves towards the HE it compresses the gas on the HE while creating suction on the CE. The process cycles reciprocally.
Some currently employed pistons are heavy enough to have speed restrictions. The speed restrictions may prevent operators from operating the compressors at their full rated speed.
A crosshead is the component of the drive train that is connected to both the connecting rod and the piston rod. The crosshead transfers the rotary/reciprocating motion of the connecting rod into a pure reciprocating motion for the piston rod. The heavy pistons may require extra heavy and extra light crossheads to balance a pair of opposing compressor throws.
The extra heavy crosshead may be very heavy and difficult for a mechanic to install, and it may also be very expensive. The extra light crosshead may be made from a material that requires special processes to manufacture and can be difficult to control the quality.
If the piston is of a multi-piece construction the piston ribs may need to be preloaded in tension to provide enough clamping force at the O.D. to prevent the O.D. contact from moving (relative motion between the two surfaces) causing contact damage. Contact damage constitutes any kind of galling, fretting, erosion of material or the like (surface failure). To preload the piston ribs, a small, approximately 0.010 inch gap, for example, may be machined into the hub between two mating surfaces so that when the piston nut is tightened the gap is closed creating a clamping force at the O.D. The preload creates a tensile stress in the ribs which must be accounted for in the fatigue safety factor. In other words, the tensile stress in the ribs from preload makes the piston weaker in fatigue.
Pistons are typically designed after the cylinder has been designed and the distance between crank end (CE) head and head end (HE) head has been established. Sometimes it is a struggle to fit enough piston rings and wear bands on the piston, and other times there is too much piston length for the recommended number of piston rings and the appropriate width of wear band. In the situation where there is excessive length on the piston engineers typically put extra piston rings on the piston. It is assumed (but not verified) that each piston ring breaks between 30% to 50% of the differential pressure to which it is exposed.
It is known that conventional, hollow, double-acting pistons fill with gas. The gas works its way inside the hollow piston over time through the asperities on the contact surfaces of the mating pieces of the piston. The gas pressure inside the piston is assumed to be between suction and discharge pressure (mean pressure). Gas pressure filling the inside of the piston is accounted for in the fatigue safety factor calculation.
Further, conventional hollow, double-acting pistons may have an o-ring seal at the O.D. that is intended to prevent oil/liquids from filling the piston. If oil/liquids fill the piston, the oil/liquids will cause an imbalance in reciprocating weight between a pair of opposing throws resulting in undesirable vibration. Oil/liquids that get inside the piston tend to stay inside the piston causing excessive vibration.